Recently, mobile communication apparatuses such as cellular phones and portable information terminals have been rapidly distributed due to development of a mobile communication system. Thus, there are increasing demands for miniaturization and high performance of the apparatuses and their components. In addition, two kinds, namely analog and digital type wireless communication systems are used for cellular phones, and a frequency for the wireless communication varies from a band of 800 MHz˜1 GHz to a band of 1.5 GHz to 2.0 GHz.
A dielectric resonator duplexer has been generally used as an antenna duplexer for the mobile communication system in consideration of low loss, power efficiency and temperature stability.
However, an SAW (Slit Acoustic Wave) duplexer is newly recommended on the basis of recent low loss design of an SAW filter, development of a power efficiency material, and development of a medium having a stabilized temperature property.
When the dielectric duplexer is compared with the SAW duplexer, the SAW duplexer has equal or more excellent properties to/than the dielectric duplexer, except for power efficiency. Especially, the SAW duplexer is absolutely advantageous in shape and size. Nevertheless, the SAW duplexer is still more expensive than the dielectric resonator type duplexer. When mass production of the SAW duplexer is achieved according to the property of an SAW manufacturing process using a semiconductor process, the SAW duplexer will have a competitive price.
FIG. 1 is a schematic diagram illustrating a general SAW filter. As illustrated in FIG. 1, the SAW filter includes a single crystal medium 101, an input inter-digital transducer (IDT) and an output IDT 103.
When the single crystal medium 101 is a piezoelectric single crystal medium such as quartz, LiTaO3 and LiNbO3, the input IDT 102 and the output IDT 103 are comprised of thin metal membranes.
In FIG. 1, an electric signal transmitted to the input IDT 102 is transduced into a mechanical wave by the piezoelectric single crystal medium 101, and propagated to the output IDT 103 through the single crystal medium 101. The wave transmitted to the output IDT 103 is re-transduced into an electric signal according to piezoelectric effects, and then outputted.
That is, the SAW filter generally used in a mobile communication terminal for filtering high frequency signals is a manual device for selectively passing wanted frequency signals by patternizing a transducer on the piezoelectric single crystal medium with thin metal membranes, and connecting the transducer to I/O terminals.
A frequency response total transfer function of the resonator of the SAW filter is provided as a composite function of material properties such as physical properties of a piezoelectric material, purity of a crystal and properties of a thin metal membrane, and device variables including variables considered in electrode design.
There have been known that a limit of a minimum value of a resonance frequency range is determined by a size of the device, and that a limit of a maximum value thereof is influenced by a line width of an electrode and loss of electric wave. Because the resonator has a very narrow bandwidth frequency response and a long impulse response, it can embody wanted properties by precisely manufacturing an electrode according to an electrode design.
As described above, the properties of the SAW device are intended to be applied to the other application fields, especially in the sensor field.